Tempered glass is glass which results from the controlled cooling of glass to impart residual internal stresses wherein the surface areas are in compression and the internal areas are in tension. The compressive stresses near the surface of the glass serve to increase the strength of the glass sheet. When the glass breaks, the unbalanced stresses cause the glass sheet to break into small, relatively harmless fragments. Personal exposure to these small glass fragments is less likely to cause severe injury than exposure to the typically larger, more jagged fragments from an untempered glass sheet. For this reason, tempered glass is utilized in most automotive glass applications other than windshields.
In order to ensure adequate safety to the end user of automotive glass products, safety codes are becoming increasingly stringent in specifying the maximum size and configuration of fragments that are permissible upon the fracturing of tempered glass sheets. Whether a particular glass sheet can meet these standards is usually determined by the degree of temper in the glass.
In a typical tempering operation, the glass sheet is heated to a temperature above its strain point and near the glass softening temperature, and then rapidly chilled by simultaneously applying blasts of cold air against each of the opposing two surfaces of the sheet. Cooling the glass in this manner results in the outside surface of the glass being cooled below its strain point before the interior of the glass, consequently placing the surface areas of the glass in compression and the interior of the glass in tension.
There is in the automotive industry an ongoing desire to reduce the weight of components in the automobile which, in turn, has led to an increasing demand for thinner automotive glass. Unfortunately, thinner glass is more difficult to effectively temper, as the glass must be cooled more quickly to impart adequate stresses. There is, therefore, a need to develop more effective tempering techniques to provide glass components having the mechanical characteristics necessary to meet these requirements.
In glass tempering processes, the glass sheet may be positioned either vertically or horizontally. U.S. Pat. No. 4,888,038 discloses one such process, known as the horizontal press bending technique, wherein the glass is oriented horizontally and is tempered immediately after being press bent to utilize the residual heat in the sheet following bending. This technique generally includes first heating the pre-trimmed sheets of glass to their softening temperature by advancing them through a heating furnace. The heated glass sheets then travel to a bending area, where they are disposed between a pair of opposed complimentary mold members and pressed into parts with a desired shape and curvature. The shaped heated glass parts are then advanced from the bending area to the tempering area, where the glass is quickly chilled to below the annealing temperature of glass.
The tempering section typically includes two opposed blastheads disposed on opposite sides of the path of movement of the glass sheets. Each blasthead is provided with a plurality of tubes or nozzles operable to direct opposed streams of cooling fluid, such as air, toward and against the opposite surfaces of the glass sheet. Thus, for example, the horizontal press forming and tempering technique described above utilizes a pair of blastheads, one disposed above and one disposed below the path of the glass. The jets of air, when directed toward the two opposing surfaces of the glass sheet, quickly cool the glass, producing the desired temper. Each blasthead typically includes a source of air under pressure as well as a plurality of tubes positioned to direct streams of the pressurized air toward and against the surfaces of the glass sheets.
In designing blastheads for large scale production processes, various technical difficulties must be overcome. For example, it is well known that the density of air varies with altitude. A decrease in air density requires a corresponding increase in total air flow to achieve the proper amount of temper in the glass. Therefore, in designing the blastheads, attention must be directed to, among other things, the geographical location at which the tempering process is to be located, i.e., whether it will be, for example, at sea level or an altitude of five thousand feet.
In addition, because the tubes of the blastheads are oriented perpendicular to the surface of the glass, the impinging air is generally directed toward the glass at about a ninety degree angle. After contacting the glass surface the impinging air flows from the interior regions of the glass sheet towards the edges, creating an air flow transverse to the impinging jets, with linearly increasing velocity toward the edges of the sheet. As a result, the impinging air jets away from the central region of the glass are deflected toward the edges, causing a reduction in heat transfer near the edges. This change in heat transfer lessens the cooling effect in these areas, and consequently modifies the temper pattern. Thus, in conventional tempering operations utilizing blastheads, the temperature pattern across the glass is modified due to the effect of the escaping air. By altering the design of the blasthead and the positioning of the tubes, more or less cooling air can be applied to areas of the glass sheet that require greater or lesser tempering, respectively. In order to aid in the designing of more efficient blastheads, it would be advantageous to be able to analyze the heat transfer pattern created by a particular blasthead in an adjacent sheet.
Monitoring of glass temperatures could be accomplished by placing thermocouples at various locations on the surface of the glass sheet. However, attaching thermocouples in this way is a tedious, time consuming process, and information gathered by this technique is limited, since a thermocouple will only give a temperature for the point at which it contacts the glass. Therefore, to obtain a clear picture of temperatures over the entire glass surface, many thermocouples, spaced closely together, would have to be attached.
An alternative to using thermocouples for temperature measurement is the use of an infrared detection device, such as, for example, an infrared camera. The use of infrared cameras measuring radiant energy is generally well known as a method for determining temperatures. For example, U.S. Pat. No. 4,818,118 discloses use of a scanning infrared radiometer (also known as an IR camera) to measure the thickness of thermal barrier coatings. The thermal barrier coating is first heated by a laser source. Then, using an infrared camera, the radiant thermal energy of a region outside the laser strike region is measured at a predetermined time following termination of the heating pulse. The intensity of this measured radiant energy is compared with the radiant energy intensities which have been experimentally obtained from specimens of known thickness, and the thickness of the coating in question is inferred therefrom.
An infrared camera could similarly be used to obtain temperature information from a heated glass sheet. However, using an infrared camera to observe a sheet of hot glass in an actual production tempering process would be difficult if not impossible due to the hostile environment and the lack of space available for observation. It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would be assembled absent the motivation provided by the present invention.